Ah, Polygroups, thanks! That's just what I was looking for :-)

I'll have another look at the Move Topological brush too. I did actually try that, before reverting to Hexagon. But I've since realised that my issue with the MT brush would have been solved by scaling up some more first... I'd not been able to reduce the size of the brush sufficiently to move just the polys I needed to, to close the lips.